Coherent laser radar device

ABSTRACT

An object of the invention is to obtain a coherent laser radar device that provides one system of a photo detector and eliminates a blind zone, including a pulsed laser that oscillates at a frequency which is identical with or close to an output light of a local light source as a single frequency; a transmission/reception optical system that irradiates a pulsed laser beam from the pulsed laser toward a target as a transmission light and receives a scattered light from the target as a reception light; a light coupling means that couples the output light from the local light source and the reception light; a photo detecting portion that conducts light coherent detection on a coupled light; and a signal processing device that calculates a speed and a distance of a target in accordance with an output of the detection, in which the photo detecting portion comprises: a photo detecting element that conducts the light coherent detection; a microwave switch that changes over a propagation path of an output from the photo detecting element; a microwave amplifier; and a switch control means that changes over the microwave switch so as to transmit a signal before a reference time as a monitor signal and transmit a signal after the reference time as a reception signal with a time at which the pulse light from the pulsed laser has completely passed through the transmission/reception optical system as the reference time.

COHERENT LASER RADAR DEVICE

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP01/03956 which has an Internationalfiling date of May 11, 2001, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a laser radar device, and moreparticularly to a coherent laser radar device using a pulsed laser thatoscillates at a single wavelength as a light source for the purpose ofmeasuring physical information such as a distance, a velocity, a densitydistribution or a velocity distribution of a target.

BACKGROUND ART

A coherent laser radar device using a laser beam can measure a windvelocity or a wind velocity distribution even in fine weather because asufficient scattering intensity is obtained even through aerosolexisting in the atmospheres. For that reason, the coherent laser radardevice is preferably located at an airport or mounted on an aircraft andexpected as a device for detecting a hindrance including an airturbulence.

The coherent laser radar devices are of two types one of which employs apulsed laser that oscillates at a single frequency as a light source andthe other of which employs a CW (continuous wave) laser.

FIG. 9 is a structural diagram showing a laser radar device in which acoherent laser radar device using an injection seeding pulsed laserdevice as a light source as disclosed in U.S. Pat. No. 5,237,331 B bySammy W. Henderson et al. is combined with a wavelength synchronizingcircuit for stabilizing the wavelength of the laser radar light sourceas disclosed in JP 10-54760 A by Shoji and Hirano.

The laser radar device shown in FIG. 9 includes a CW laser light source1 that oscillates at a single frequency, a first optical divider 2 thatbranches a laser beam 19 from the CW laser light source 1, a frequencyshifter 3, an injection seeding pulsed laser 4, a beam splitter 5, a ¼wavelength plate 6, a telescope 7, a scanning optical system 8, a firstoptical coupler 9, a photo detecting portion 10, a second opticaldivider 11, a third optical divider 12, a second optical coupler 13, asignal processing device 16, an adjusting mechanism 17 for a cavitylength of the injection seeding pulsed laser 4, and a control circuit 18for the adjusting mechanism. Reference numeral 20 denotes a seed lightfrom the frequency shifter 3, 21 is a pulsed laser beam from theinjection seeding pulsed laser 4, 22 is an optical axis of atransmit/reception light, 23 is a transmission light, 24 is a receptionlight, 25 is a local light and 26 is a coupled light of the receptionlight 24 and the local light 25 due to the first optical coupler 9.

Subsequently, the operation of the laser radar device shown in FIG. 9will be described. The laser beam 19 from the laser light source 1 thatoscillates at a single frequency f₀ is branched by the first opticaldivider 2 into two beams one of which forms the local light 25, and theother of which becomes a laser beam that increases in frequency by afrequency f_(IF) by the frequency shifter 3 and is then supplied to theinjection seeding pulsed laser 4 as the seed light 20.

The injection seeding pulsed laser 4 conducts the pulse oscillation atthe single frequency (single wavelength) in an axis mode having afrequency closest to the seed light 20. The laser pulse 21 from theinjection seeding pulsed laser 4 which is linearly polarized isreflected by the beam splitter 5 through the second optical divider 11.Thereafter, the reflected light is transformed into a circularlypolarized light by the ¼ wavelength plate 6 and then irradiated toward atarget through the telescope 7 and the scanning optical system 8 as thetransmission light 23.

A scattered light from the target is received through a backward path ofthe transmission light. The reception light 24 becomes a linearlypolarized light shifted from a polarization plane of the laser pulse 21by 90 degrees due to the ¼ wavelength plate 6 and is then transmittedthrough the beam splitter 5 so as to be guided to the first opticalcoupler 9. In the first optical coupler 9, the reception light 24 andthe local light 25 are coupled together, and the coupled light 26 issupplied to the photo detecting portion 10.

In this example, the photo detecting portion 10 is structured as shownin FIG. 10.

As shown in FIG. 10, the photo detecting portion 10 includes a firstphoto detector 27 and a second photo detector 28. Each of the first andsecond photo detectors 27 and 28 is made up of a photodiode thatfunctions as a square-law detector which conducts light coherentdetection and a microwave amplifier that electrically amplifies a signalfrom the photodiode. The microwave amplifier is shown by the combinationof a pre-amplifier and a post-amplifier in the figure. A detectionoutput from the first photo detector 27 is outputted to the signalprocessing device 16 as a reception signal, and a detection output fromthe second photo detector 28 is outputted to the signal processingdevice 16 as a monitor signal.

Returning to FIG. 9, the coupled light 26 from the first optical coupler9 is coherent-detected by the first photo detector 27 of the photodetecting portion 10. A signal from the first photo detector 27 isinputted to the signal processing device 16 as the reception signal. Thesignal processing device 16 calculates a distance to the target inaccordance with an arrival period of time of the reception signal (aperiod of time since the transmission of the transmission light to thetarget till the reception of the reception light from the target),analyzes the frequency of the reception signal to obtain a Dopplersignal, and extracts the velocity of the target from the Doppler signal.

As described above, the injection seeding pulsed laser 4 is required tomonitor a difference in frequency between the pulsed laser beam 21 andthe local light 25 in order to obtain an accurate Doppler signal sincethe injection seeding pulsed laser 4 conducts the pulse oscillation atthe single frequency in an axis mode having a frequency closest to theseed light 20. For that reason, after a part of the laser pulse 21 and apart of the local light 25 are extracted as monitor lights from thesecond and third optical dividers 11 and 12, respectively, and thencoupled together by the second optical coupler 13, the coherentdetection is conducted by the second photo detector 28 within the photodetecting portion 10 a. A signal from the second detector 28 becomes themonitor signal.

In the signal processing device 16, a frequency difference (thefrequency of the monitor signal) f_(M) between the laser pulse 21 andthe local light 25 and the oscillation timing of the laser pulse areobtained from the monitor signal. Assuming that the frequency of thelocal light 25 is f₀, the respective frequencies f_(s), f_(T), f_(R),f_(M) and f_(sig) of the seed light, the laser pulse, the receptionlight, the monitor signal and the reception signal are represented bythe following expressions.

f_(s)=f₀+f_(IF)

f_(T)=f_(s)+Δf

f_(R)=f_(T)+f_(d)

f_(M)=f_(IF)+Δf

f_(sig)=f_(M)+f_(d)

where Δf is a frequency difference between the laser pulse 21 and theseed light 20, and f_(d) is a Doppler frequency of the target. Adifference between the frequency f_(sig) of the reception signal and thefrequency f_(M) of the monitor signal is taken, thereby being capable ofobtaining the Doppler frequency f_(d) of the target.

In order that the injection seeding pulsed laser 4 stably obtains theinjection seeding operation, the injection seeding pulsed laser 4adjusts the cavity length of the pulsed laser by using a piezoelectricelement as the adjusting mechanism 17 for the cavity length. Thepiezoelectric element that functions as the adjusting mechanism 17 ofthe cavity length is controlled by the control circuit 18. In the signalprocessing device 16, an error signal based on a value of the frequencydifference f_(M) between the laser pulse 21 and the local light 25 istransmitted to the control circuit 18 from the monitor signal. In thecontrol circuit 18, the cavity length of the pulsed laser 4 is adjustedby the piezoelectric element so that the value of Δf is set to be a setvalue or less, or 0.

In this way, the laser pulse that stably oscillates in a single mode(single wavelength) is obtained.

FIG. 11 is a block diagram showing a structure of a signal processingdevice 16 a as an example of the signal processing device 16. The signalprocessing device 16 a includes a first frequency discriminator 101, asecond frequency discriminator 102, a third frequency discriminator 103and an arithmetic operation device 104.

The first frequency discriminator 101 conducts a frequency analysis uponreceiving a reception signal from the first photo detector 27 andextracts a Doppler frequency from a target. The second frequencydiscriminator 102 conducts the frequency analysis of the monitor signaland obtains the frequency difference f_(M) between the laser pulse 21and the local light 25 and the oscillation timing of the laser pulsefrom the monitor signal. The third frequency discriminator 103 transmitsthe error signal based on the value of the frequency difference f_(M)between the laser pulse 21 and the local light 25 to the control circuit18 from the monitor signal. The arithmetic operation device 104calculates a distance and a velocity of the target on the basis of anoutput signal from the first and second frequency discriminators 101 and102.

In this example, the structure of the frequency discriminator thatfunctions as the first frequency discriminator 101, the second frequencydiscriminator 102 and the third frequency discriminator 103 includes anA/D converter that converts the reception signal or the monitor signalinto a digital signal and a signal processing portion that processes thedigital signal converted by the A/D converter into a necessary signal bya frequency analyzing means such as a fast Fourier transform (FFT) asshown in FIG. 12.

Also, as shown in FIG. 13, the frequency discriminator may be made up ofan electric filter portion which is made up of one or a plurality ofelectric filters, and a signal processing portion that conducts anecessary signal processing in accordance with the transmittance of asignal from the electric filter portion.

Also, the signal processing device 16 can employ a signal processingdevice 16 b that incorporates the function of the third frequencydiscriminator 103 shown in FIG. 11 into the second frequencydiscriminator 102 shown in FIG. 11, as shown in FIG. 14, and has thesame function as that of the signal processing device 16 a shown in FIG.11.

As described above, in the photo detecting portion 10 of the coherentlaser radar device using the conventional injection seeding pulsed laser4 as the light source, the second photo detector 28 for monitoring theoscillation frequency of the injection seeding pulsed laser 4 isdisposed in addition to the first photo detector 27 that detects thereception light as shown in the photo detecting portion 10 a shown inFIG. 10. In addition, at least two systems for the reception signal, themonitor and so on are prepared for the frequency discriminator as shownin FIGS. 11 and 14.

In the case where the intensity of the monitor light is sufficientlylarge, a part or the entire microwave amplifier of the second photodetector 28 can be omitted.

Subsequently, an influence of an internal reflection light of thecoherent laser radar device using a pulsed laser light source and acoaxial transmission/reception optical system as shown in FIG. 9 will bedescribed.

In FIG. 9, the beam splitter 5, the ¼ wavelength plate 6, the telescope7 and the scanning optical system 8 are of the coaxialtransmission/reception optical system that makes the optical axes 22 ofthe transmit/reception lights substantially coincide with each other.

In the coherent laser radar device using the coaxialtransmission/reception optical system of this type, the internalreflection lights from the optical elements that constitute the coaxialtransmission/reception optical system reach the photo detecting portion10 through the same path as that through which the reception lightpasses. In particular, since the internal reflection lights from thetelescope 7 and the scanning optical system 8 pass through the beamsplitter 5 as a reception side, the influence of the internal reflectionlight is large. As usual, the attenuation of reflection of the telescope7 and the scanning optical system 8 is about 60 to 70 dB. On thecontrary, the attenuation of reflection of the reception light fromaerosol contained in the atmosphere exceeds 100 dB.

In order to conduct a high-precision measurement, the microwaveamplifier which is made up of the pre-amplifier and the post-amplifierof the first photo detector 27 within the photo detecting portion 10 ashown in FIG. 10 is required to amplify the reception signal up to abouta degree suitable for the maximum sampling amplitude of the firstfrequency discriminator 101. Since the reception light is slight, themicrowave amplifier of the first photo detector 27 has a high gain.Since the internal reflection light is much larger in power than thereception light, the internal reflection light induces the saturation ofthe microwave amplifier in the first photo detector 27. Since the linearamplification of the signal is not conducted until the microwaveamplifier is restored since the microwave amplifier is saturated, themeasurement cannot be conducted. As usual, it takes several μs until theinfluence of such an internal reflection light is eliminated. For thatreason, a “blind zone” where the short distance of several hundreds of mfrom the device cannot be measured occurs.

As described above, the coherent laser radar device using theconventional injection seeding pulsed laser 4 shown in FIG. 9 as thelight source and also using the coaxial transmission/reception opticalsystem suffers from the following drawbacks.

1. In order to monitor the oscillation frequency of the injectionseeding pulsed laser 4, the photo detector for monitoring is disposed inaddition to the photo detector that detects the reception light,resulting in the complicated photo detecting portion.

2. Likewise, at least two systems for the reception signal and themonitor are disposed for the frequency discriminator with the resultthat the signal processing device is complicated.

3. The wide “blind zone” where the measurement cannot be conducted overthe short distance of several hundreds of m from the device occurs dueto the influence of the internal reflection light of thetransmission/reception optical system.

The present invention has been made to eliminate the above-describedproblems, and therefore an object of the present invention is to providea coherent laser radar device using an injection seeding pulsed laser asa light source and also using a coaxial transmission/reception opticalsystem in which a photo detector is of one system and the blind zone canbe eliminated.

DISCLOSURE OF THE INVENTION

In order to achieve the above-mentioned object, according to the presentinvention, there is provided a coherent laser radar device, comprising:a local light source that oscillates at a single frequency; a pulsedlaser that oscillates at a frequency which is identical with or close toan output light of the local light source as a single frequency; atransmission/reception optical system that irradiates a pulsed laserbeam from the pulsed laser toward a target as a transmission light andreceives a scattered light from the target as a reception light; a lightcoupling means that couples the output light from the local light sourceand the reception light; a photo detecting portion that conducts lightcoherent detection on the light coupled by the light coupling means; anda signal processing device that calculates a speed and a distance of atarget in accordance with an output from the photo detecting portion,characterized in that the photo detecting portion comprises: a photodetecting element that conducts the light coherent detection; amicrowave switch that changes over a propagation path of an output fromthe photo detecting element; a microwave amplifier; and a switch controlmeans that changes over the microwave switch so as to transmit a signalbefore a reference time to the signal processing device as a monitorsignal and transmit a signal after the reference time to the signalprocessing device as a reception signal with a time at which the pulselight from the pulsed laser has completely passed through thetransmission/reception optical system as the reference time.

Also, the pulsed laser includes an adjusting mechanism that adjusts acavity length, the device further comprises a control circuit thatcontrols the adjusting mechanism, and the control circuit outputs to theadjusting mechanism a control signal that adjusts the cavity length ofthe pulsed laser on the basis of an error signal from the signalprocessing device based on a frequency difference between the laserpulse and the local light.

Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the microwave switchis disposed between the pre-amplifier and the post-amplifier, outputs asignal that has been amplified by the pre-amplifier as a monitor signaland outputs a signal that has passed through the post-amplifier as areception signal.

Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the microwave switchis disposed between the photo detecting element and the pre-amplifier,outputs a signal from the photo detecting element as a monitor signaland outputs a signal that has passed through the post-amplifier as areception signal.

Also, according to another aspect of the present invention, there isprovided a coherent laser radar device, comprising: a local light sourcethat oscillates at a single frequency; a pulsed laser that oscillates ata frequency which is identical with or close to an output light of thelocal light source as a single frequency; a transmission/receptionoptical system that irradiates a pulsed laser beam from the pulsed lasertoward a target as a transmission light and receives a scattered lightfrom the target as a reception light; a light coupling means thatcouples the output light from the local light source and the receptionlight; a photo detecting portion that conducts light coherent detectionon the light coupled by the light coupling means; and a signalprocessing device that detects a speed and a distance of a target inaccordance with an output from the photo detecting portion,characterized in that the photo detecting portion comprises: a photodetecting element that conducts the light coherent detection; amicrowave amplifying portion that amplifies an output signal from thephoto detecting element; and a gain control means that controls the gainof the microwave amplifying portion so that an amplitude of the outputsignal from the microwave amplifying portion does not exceed a giventhreshold value.

Also, the pulsed laser includes an adjusting mechanism that adjusts acavity length; the device further comprises a control circuit thatcontrols the adjusting mechanism, and the control circuit outputs to theadjusting mechanism a control signal that adjusts the cavity length ofthe pulsed laser on the basis of an error signal from the signalprocessing device based on a frequency difference between the laserpulse and the local light.

Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a gain controlamplifier that amplifies an output of the pre-amplifier, and the gaincontrol means controls the gain of the gain control amplifier.

Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the gain controlmeans comprises a microwave variable attenuator disposed between thepre-amplifier and the post-amplifier, and an attenuation control circuitthat controls the attenuation of the microwave variable attenuator.

Also, a coherent laser radar device according to still another aspect ofthe present invention is characterized by comprising: a local lightsource that oscillates at a single frequency; a pulsed laser thatoscillates at a frequency which is identical with or close to an outputlight of the local light source as a single frequency; atransmission/reception optical system that irradiates a pulsed laserbeam from the pulsed laser toward a target as a transmission light andreceives a scattered light from the target as a reception light; a lightcoupling means that couples the output light from the local light sourceand the reception light; a photo detecting portion that conducts lightcoherent detection on the light coupled by the optical coupler; and asignal processing device that detects a speed and a distance of a targetin accordance with an output from the photo detecting portion; a lightvariable attenuator disposed between the transmission/reception opticalsystem and the photo detecting portion; and a control means thatcontrols the attenuation of the light variable attenuator in such amanner that an amplitude of the output from the photo detecting portiondoes not exceed a given threshold value.

Further, the pulsed laser includes an adjusting mechanism that adjusts acavity length, the device further comprises a control circuit thatcontrols the adjusting mechanism, and the control circuit outputs to theadjusting mechanism a control signal that adjusts the cavity length ofthe pulsed laser on the basis of an error signal from the signalprocessing device based on a frequency difference between the laserpulse and the local light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a coherent laser radardevice in accordance with a first embodiment of the present invention;

FIG. 2 is a block diagram showing a structure of a photo detectingportion 10Aa employed as a photo detecting portion 10A in the firstembodiment of the present invention;

FIG. 3 is a block diagram showing a structure of a photo detectingportion 10Ab employed as a photo detecting portion 10A in the firstembodiment of the present invention;

FIG. 4 is a block diagram showing a structure of a photo detectingportion 10B in accordance with a second embodiment of the presentinvention;

FIG. 5 is a block diagram showing a structure of a signal processingdevice 16A used as a signal processing device 16 in accordance with thesecond embodiment of the present invention;

FIG. 6 is a block diagram showing a structure of a photo detectingportion 10C in accordance with a third embodiment of the presentinvention;

FIG. 7 is a block diagram showing a structure of a coherent laser radardevice in accordance with a fourth embodiment of the present invention;

FIG. 8 is a block diagram showing a structure of a photo detectingportion 10D and an optical system immediately in front of the photodetecting portion 10D in the fourth embodiment of the present invention;

FIG. 9 is a block diagram showing a structure of a laser radar device inwhich a coherent laser radar device using an injection seeding pulsedlaser device as a light source as disclosed in U.S. Pat. No. 5,237,331 Bis combined with a wavelength synchronizing circuit for stabilizing thewavelength of a laser radar light source as disclosed in JP 10-54760 A;

FIG. 10 is a block diagram showing a structure of a photo detectingportion 10 a in accordance with a conventional example;

FIG. 11 is a block diagram showing a structure of a signal processingdevice 16 a as an example of a signal processing device 16 in accordancewith a conventional example;

FIG. 12 is a block diagram showing an example of a structure of afrequency discriminator in FIG. 11;

FIG. 13 is a block diagram showing another example of the structure ofthe frequency discriminator in FIG. 11; and

FIG. 14 is a block diagram showing the structure of a signal processingdevice 16 b as an example of a signal processing device 16 in theconventional example.

BEST MODES FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a block diagram showing a structure of a coherent laser radardevice in accordance with a first embodiment of the present invention.In FIG. 1, the same parts as those in the conventional example shown inFIG. 9 are designated by like reference numerals and their descriptionwill be omitted.

In the coherent laser radar device shown in FIG. 1, differences from theconventional example shown in FIG. 9 will be described. First, there isdisposed a photo detecting portion 10A that is different in the internalstructure from the conventional example as will be described later.Also, there is disposed a coupling optical system 29 between the beamsplitter 5 and the first optical coupler 9. In addition, the laser lightsource 1, the first optical divider 2, the frequency shifter 3 and theinjection seeding pulsed laser 4 are connected to each other in thestated order by optical fibers, the first optical divider 2 and thefirst optical coupler 9 are connected to each other by an optical fiber,and the coupling optical system 29, the first optical coupler 9 and thephoto detecting portion 10A of the first embodiment are connected toeach other by optical fibers in the stated order. Then, the receptionlight 24 is coupled to the optical fiber by the coupling optical system29 and then coupled with the local light 25 in the fiber inline typeoptical coupler 9. The coupled light 26 consisting of the receptionlight 24 and the local light 25 passes through the optical fiber and arethen supplied to the photo detecting portion 10A.

On the other hand, the same portions as those in the conventionalexample shown in FIG. 9 will be described. A process that the injectionseeding pulsed laser (hereinafter simply referred to as pulsed laser) 4oscillates at a single frequency and a process that an output laserpulse from the injection seeding pulsed laser is transmitted toward atarget as a transmission light and a scattered light from the target isreceived are identical with those in the conventional example shown inFIG. 9. Also, a process from supplying of the monitor signal and thereception signal to the signal processing device 16 from the photodetecting portion 10A of the first embodiment to the signal processingin the signal processing device 16 are also identical with those in theconventional example shown in FIG. 9. The signal processing device 16may be structured by the signal processing device 16 a shown in FIG. 11or the signal processing device 16 b shown in FIG. 14.

FIG. 2 is a block diagram showing a structure of a photo detectingportion 10Aa employed as a photo detecting portion 10A in the firstembodiment of the present invention. The photo detecting portion 10Aashown in FIG. 2 is made up of a photodiode 30, a pre-amplifier 31, apost-amplifier 32, a microwave switch 33 and a switch control circuit 34that controls the microwave switch 33.

The microwave switch 33 is controlled by the switch control circuit 34as follows. While an internal reflection light is received from thetransmission/reception optical system to the photodiode 30 before thepulsed laser 4 oscillates, the microwave switch 33 transmits a signalfrom the photodiode 30 which has been amplified by the pre-amplifier 31to the signal processing device 16 as the monitor signal. After theinternal reflection light has been sufficiently attenuated, themicrowave switch 33 transmits a signal from the photodiode 30 to thesignal processing device 16 through the post-amplifier 32 as thereception signal. That is, the switch control circuit 34 changes overthe microwave switch so as to transmit a signal before a reference timeto the signal processing device 16 as the monitor signal and transmit asignal after the reference time to the signal processing device 16 asthe reception signal with a time at which the pulse light from thepulsed laser 4 has completely passed through the transmission/receptionoptical system as the reference time.

Since the internal reflection light is also a part of the output laserpulse of the pulsed laser 4, the light coherent detection signal can beemployed as a monitor signal for obtaining the frequency differencebetween the laser pulse 21 and the local light 25 and the oscillationtiming of the laser pulse. The microwave switch 33 changes over afterthe internal reflection light has been sufficiently attenuated, therebybeing capable of transmitting the reception signal without saturatingthe post-amplifier 32.

In addition, if a timing at which the microwave switch 33 changes overis a time point at which a signal resulting from the internal reflectionlight is attenuated to the degree at which the post-amplifier 32 is notsaturated, the “blind zone” can be reduced. For example, if the pulsewidth of the laser pulse 21 is 200 ns, it is possible to set thechange-over timing to 1 μs or less after oscillation. That is, the“blind zone” can be reduced to 150 m or less.

If the peak intensity of the internal reflection light is sufficientlyhigh, the microwave switch 33 may be disposed between the photodiode 30and the pre-amplifier 31 as in the photo detecting portion 10Ab shown inFIG. 3. In this situation, the timing at which the microwave switch 33changes over is a point of time where the internal reflection lightattenuates to the degree at which the pre-amplifier 31 and thepost-amplifier 32 are not saturated.

The microwave switch 33 is required to provide the switching speed ofabout 0.1 μs and sufficient In-Out isolation that does not saturate theamplifier when the signal is off. However, in a semiconductor switchusing GaAs or the like, the switch speed of 10 ns and the In-Outisolation of 40 dB or more are realized, and the microwave switch 33 canbe formed of the semiconductor switch.

Since the above-mentioned structure makes it possible that the microwaveswitch 33 changes over to produce the monitor signal by using theinternal reflection light, it is unnecessary to additionally provide anoptical system for extracting a part of the laser pulse 24 and a photodetector for producing the monitor signal with the result that thedevice can be simplified. Also, the “blind zone” can be reduced by thechange-over timing of the microwave switch 33 without saturating thepre-amplifier 31 and the post-amplifier 32.

Second Embodiment

FIG. 4 is a block diagram showing a structure of a photo detectingportion 10B in accordance with a second embodiment of the presentinvention. The photo detecting portion 10B in the second embodiment ofthe present invention is employed instead of the photo detecting portion10A according to the first embodiment in FIG. 1 and includes aphotodiode 30, a pre-amplifier 31, a gain control amplifier 35 and again control circuit 36 that controls the gain of the gain controlamplifier 35.

The pre-amplifier 31 and the gain control amplifier 35 structure themicrowave amplifier in the photo detecting portion 10B, and the gaincontrol amplifier 35 and the gain control circuit 36 structure the gaincontrol means of the microwave amplifier.

Also, FIG. 5 is a block diagram showing a signal processing device 16Aused as a signal processing device 16 in accordance with the secondembodiment of the present invention. Since the signal processing device16A in the second embodiment of the present invention is employed alsoas the signal processing device 16 shown in FIG. 1, the signalprocessing device 16A is made up of only a frequency discriminator 110and an arithmetic operation device 104.

In the second embodiment, a process that the injection seeding pulsedlaser 4 oscillates at a single frequency and a process that an outputlaser pulse from the injection seeding pulsed laser is transmittedtoward a target as a transmission light and a scattered light from thetarget is received and supplied to the photo detecting portion 10B areidentical with those in the first embodiment. The second embodiment ischaracterized in that the gain of the microwave amplifier in the photodetecting portion 10B is controlled in accordance with a time. The gainof the microwave amplifier is set to be low so that the peak value of asignal amplitude resulting from the internal reflection light does notexceed the sampling maximum amplitude of the frequency discriminator 110while the internal reflection light is received by the photodiode 30from the transmission/reception optical system. Thereafter, after theinternal reflection light has been sufficiently attenuated, the gain ofthe microwave amplifier is set to a high gain sufficient to amplify thereception signal to the degree suitable for the sampling maximumamplitude of the frequency discriminator 110.

As described above, the gain of the microwave amplifier is controlled insuch a manner that the monitor signal can enter a time zone of thereception signal which has not been conventionally used in the signalprocessing as the “blind zone”. Since the monitor signal and thereception signal are allowed to flow in one signal line in a timedivision manner, two systems of the photo detectors and the frequencydiscriminators which have been required as the reception signal and themonitor signal up to now can be simplified into one system. In addition,there is advantageous in that the “blind zone” can be reduced due to thechange-over timing of the gain of the microwave amplifier as in thefirst embodiment.

The above-mentioned structures make it possible to obtain an advantagethat two systems of the photo detectors and the frequency discriminatorswhich have been required as the reception signal and the monitor signalup to now can be simplified into one system and an advantage that the“blind zone” can be reduced due to the change-over timing of the gain ofthe microwave amplifier as in the first embodiment.

Third Embodiment

FIG. 6 is a block diagram showing a photo detecting portion 10C inaccordance with a third embodiment of the present invention. The photodetecting portion 10C in the third embodiment of the present inventionis used instead of the photo detecting portion 10A according to thefirst embodiment in FIG. 1 and includes a photodiode 30, a pre-amplifier31, a post-amplifier 32, a microwave variable attenuator 37 and anattenuation control circuit 38 that controls the attenuation of thevariable attenuator 37.

In other words, in the photo detecting portion 10C in the thirdembodiment, the microwave variable attenuator 37 is inserted between thepre-amplifier 31 and the post-amplifier 32 to structure the microwaveamplifier of the photo detector. Then, the attenuation control circuit38 controls the attenuation of the variable attenuator 37.

As a result, the monitor signal and the reception signal are allowed toflow in one output signal line in a time division manner.

The signal processing device 16 in the third embodiment of the presentinvention employs the same signal processing device 16A as that in thesecond embodiment shown in FIG. 5.

The above-mentioned structure makes it possible to obtain an advantagethat two systems of the photo detectors and the frequency discriminatorswhich have been required as the reception signal and the monitor signalup to now can be simplified into one system and an advantage that the“blind zone” can be reduced due to the change-over timing of the gain ofthe microwave amplifier as in the first embodiment.

The variable attenuator 37 can be formed of a semiconductor switch usingGaAs or the like.

Fourth Embodiment

FIG. 7 is a block diagram showing the structure of a coherent laserradar device in accordance with a fourth embodiment of the presentinvention. In FIG. 7, the same parts as those in the first embodimentshown in FIG. 1 are designated by like references and their descriptionwill be omitted. In the coherent laser radar device according to thefourth embodiment, a variable light attenuator 39 and an attenuationcontrol circuit 40 that controls the attenuation of the variable lightattenuator 39 are further disposed between the coupling optical system27 and the first optical coupler 9 with respect to the first embodimentshown in FIG. 1, as shown in FIG. 7. Also, there is disposed a photodetecting portion 10D different in the internal structure from those inthe first to third embodiments as will be described later. The signalprocessing device 16 in the fourth embodiment of the present inventionis formed of a signal processing device 16A like to that in the secondembodiment shown in FIG. 5.

FIG. 8 is a block diagram showing the structure of a photo detectingportion 10D and an optical system immediately in front of the photodetecting portion 10D in the fourth embodiment of the present invention.

As shown in FIG. 8, in the fourth embodiment, the photo detectingportion 10D structures a pair of photo detectors formed of thecombination of a photodiode 30 that is a photo detecting element whichconducts coherent detection with a microwave amplifier which is formedof a pre-amplifier 31 and a post-amplifier 32 and has a fixed gain.

In this embodiment, the function of the structure of the secondembodiment shown in FIGS. 4 and 5 is achieved by controlling theattenuation of the reception light 24 by the variable light attenuator39 and the attenuation control circuit 40. That is, in the secondembodiment, the function with which an output signal from the photodetector 10B to the signal processing device 16B does not exceed thesampling maximum amplitude of the frequency discriminator 110 by thegain control circuit 36 of the microwave amplifier in the photo detector10B even while the internal reflection light is received by thephotodiode 30 from the transmission/reception optical system is achievedby controlling the attenuation of the reception light 24 by the variablelight attenuator 39 and the attenuation control circuit 40.

The attenuation of the variable light attenuator 39 is set to be high sothat a peak value of a signal amplitude resulting from the internalreflection light does not exceed the sampling maximum amplitude of thefrequency discriminator 110 shown in FIG. 5 while the internalreflection light is received by the photodiode 30 shown in FIG. 8 fromthe transmission/reception optical system, to thereby limit the power ofthe internal reflection light received by the photodiode 30. Thereafter,the internal reflection light has been sufficiently attenuated, controlis then made in such a manner that the attenuation of the variable lightattenuator 39 is set to 0 or nearly 0, and the power loss of thereception light which is received by the photodiode 30 is lowered. Thegain of the microwave amplifier is set to sufficient high to amplify thereception signal at that time to the degree suitable for the samplingmaximum amplitude of the frequency discriminator 110.

As described above, the attenuation of the variable light attenuator 39is controlled, to thereby make it possible that the monitor signalenters the time zone of the reception signal that has not beenconventionally used in the signal processing as the “blind zone” as inthe second embodiment. Since the monitor signal and the reception signalare allowed to flow in one signal line in a time division manner, twosystems of the photo detectors and the frequency discriminators whichhave been required as the reception signal and the monitor signal up tonow can be simplified into one system. In addition, there isadvantageous in that the “blind zone” can be reduced due to thechange-over timing of the gain of the microwave amplifier as in thesecond embodiment.

The above-mentioned structure makes it possible to obtain an advantagethat two systems of the photo detectors and the frequency discriminatorswhich have been required as the reception signal and the monitor signalup to now can be simplified into one system and an advantage that the“blind zone” can be reduced due to the change-over timing of the gain ofthe microwave amplifier as in the first embodiment.

The above-mentioned first to fourth embodiments show the structuralexamples in which the laser light source 1, the first optical divider 2,the frequency shifter 3 and the injection seeding pulsed laser 4 arecoupled to each other by optical fibers in the stated order,respectively, the first optical divider 2 and the first optical coupler9 are coupled to each other by an optical fiber, and the couplingoptical system 29, the first optical coupler 9, the photo detectingportion 10 (10A, 10Aa, 10Ab, 10B, 10C, 10D) are coupled to each other byoptical fibers in the stated order, respectively, so as to allow thelocal light 25, the reception light 24, the seed light 20 and thecoupled light 26 to be propagated in the optical fiber.

The first to fourth embodiments may be structured in such a manner thatall or a part of the coupling optical system 29 and the optical fiberare omitted as in the conventional example shown in FIG. 9, and all or apart of the local light 25, the reception light 24, the seed light 20and the coupled light 26 are propagated into a space, and the sameadvantages as those described above can be obtained.

INDUSTRIAL APPLICABILITY

As was described above, according to the present invention, in thecoherent laser radar device that uses the injection seeding pulsed laseras a light source and also uses the coaxial transmission/receptionoptical system, there can be obtained the coherent laser radar devicewhich is capable of simplifying the photo detectors of two systems forthe reception signal and the monitor signal into one system, and is alsocapable of reducing the blind zone.

What is claimed is:
 1. A coherent laser radar device comprising: a locallight source that oscillates at a single frequency; a pulsed laser thatoscillates at a frequency which is identical with or close to the singlefrequency of said local light source; a transmission/reception opticalsystem that irradiates a pulsed laser beam from said pulsed laser towarda target as a transmission light and receives a scattered light from thetarget as a reception light; a light coupling means that couples theoutput light from said local light source and said reception light; aphoto detecting portion that conducts light coherent detection on thelight coupled by said light coupling means; and a signal processingdevice that calculates a speed and a distance of a target in accordancewith an output from said photo detecting portion, characterized in thatsaid photo detecting portion comprises: a photo detecting element thatconducts the light coherent detection; a microwave switch that changesover a propagation path of an output from said photo detecting element;a microwave amplifier; and a switch control means that changes over saidmicrowave switch so as to transmit a signal before a reference time tosaid signal processing device as a monitor signal and transmit a signalafter the reference time to said signal processing device as a receptionsignal with said reference time being a time at which the pulse lightfrom said pulsed laser has completely passed through saidtransmission/reception optical system.
 2. A coherent laser radar deviceaccording to claim 1, characterized in that: said pulsed laser includesan adjusting mechanism that adjusts a cavity length; said device furthercomprises a control circuit that controls said adjusting mechanism; andsaid control circuit outputs to said adjusting mechanism a controlsignal that adjusts the cavity length of said pulsed laser on the basisof an error signal from said signal processing device based on afrequency difference between the laser pulse and the local light.
 3. Acoherent laser radar device according to claim 1, characterized in that:said microwave amplifier is made up of a pre-amplifier that amplifies asignal from said photo detecting element and a post-amplifier thatamplifies an output of the pre-amplifier; and said microwave switch isdisposed between said pre-amplifier and said post-amplifier, outputs asignal that has been amplified by said pre-amplifier as a monitor signaland outputs a signal that has passed through said post-amplifier as areception signal.
 4. A coherent laser radar device according to claim 1,characterized in that: said microwave amplifier is made up of apre-amplifier that amplifies a signal from said photo detecting elementand a post-amplifier that amplifies an output of the pre-amplifier; andsaid microwave switch is disposed between said photo detecting elementand said pre-amplifier, outputs a signal from said photo detectingelement as a monitor signal and outputs a signal that has passed throughsaid post-amplifier as a reception signal.
 5. A coherent laser radardevice comprising: a local light source that oscillates at a singlefrequency; a pulsed laser that oscillates at a frequency which isidentical with or close to the single frequency of said local lightsource; a transmission/reception optical system that irradiates a pulsedlaser beam from said pulsed laser toward a target as a transmissionlight and receives a scattered light from the target as a receptionlight; a coupling means that couples said reception light and aninternal reflection light to an optical path, the internal reflectionlight comprising a part of the pulsed laser beam from said pulsed laserused for generating a monitor signal; a photo detecting portion thatconducts light coherent detection on the light coupled to the opticalpath by said light coupling means; and a signal processing device thatcalculates a speed and a distance of a target in accordance with anoutput from said photo detecting portion, characterized in that saidphoto detecting portion comprises: a photo-detecting element thatconducts the light coherent detection; a microwave amplifying portionthat amplifies an output signal from said photo detecting element; and again control means that controls the gain of said microwave amplifyingportion so that an amplitude of the output signal from said microwaveamplifying portion does not exceed a given threshold value.
 6. Acoherent laser radar device according to claim 5, characterized in that:said pulsed laser includes an adjusting mechanism that adjusts a cavitylength; said device further comprises a control circuit that controlssaid adjusting mechanism; and said control circuit outputs to saidadjusting mechanism a control signal that adjusts the cavity length ofsaid pulsed laser on the basis of an error signal from said signalprocessing device based on a frequency difference between the laserpulse and the local light.
 7. A coherent laser radar device according toclaim 5, characterized in that: said microwave amplifier is made up of apre-amplifier that amplifies a signal from said photo detecting elementand a gain control amplifier that amplifies an output of thepre-amplifier; and said gain control means controls the gain of saidgain control amplifier.
 8. A coherent laser radar device according toclaim 5, characterized in that: said microwave amplifier is made up of apre-amplifier that amplifies a signal from said photo detecting elementand a post-amplifier that amplifies an output of the pre-amplifier; andsaid gain control means comprises a microwave variable attenuatordisposed between said pre-amplifier and said post-amplifier, and anattenuation control circuit that controls the attenuation of saidmicrowave variable attenuator.
 9. A coherent laser radar devicecharacterized by comprising: a local light source that oscillates at asingle frequency; a pulsed laser that oscillates at a frequency which isidentical with or close to the single frequency of said local lightsource; a transmission/reception optical system that irradiates a pulsedlaser beam from said pulsed laser toward a target as a transmissionlight and receives a scattered light from the target as a receptionlight; a coupling means that couples said reception light and aninternal reflection light to an optical path, the internal reflectionlight comprising a part of the pulsed laser beam from said pulsed laserused for generating a monitor signal; a photo detecting portion thatconducts light coherent detection on the light coupled to the opticalpath by said optical coupler; and a signal processing device thatdetects a speed and a distance of a target in accordance with an outputfrom said photo detecting portion; a light variable attenuator disposedbetween said transmission/reception optical system and said photodetecting portion; and a control means that controls the attenuation ofsaid light variable attenuator in such a manner that an amplitude of theoutput from said photo detecting portion does not exceed a giventhreshold value.
 10. A coherent laser radar device according to claim 9,characterized in that: said pulsed laser includes an adjusting mechanismthat adjusts a cavity length; said device further comprises a controlcircuit that controls said adjusting mechanism; and said control circuitoutputs to said adjusting mechanism a control signal that adjusts thecavity length of said pulsed laser on the basis of an error signal fromsaid signal processing device based on a frequency difference betweenthe laser pulse and the local light.
 11. An apparatus implemented in alaser radar device, the laser radar device being configured to transmita pulsed laser beam toward a target and process scattered light receivedfrom the target in a signal processing unit, the apparatus comprising:light coupling means for coupling the output light of a light sourcewith the received scattered light, the light source being used togenerate the pulsed laser beam; photo-detecting means for generating asignal by performing coherent detection on the light coupled by thelight coupling means; outputting means for outputting the signalgenerated by the photo-detecting means to the signal processing unit asa monitor signal before a reference time, and for outputting anamplified version of the signal generated by the photo-detecting meansto the signal processing unit as a reception signal after the referencetime, wherein the reception signal is used by the signal processing unitto determine at least one of a speed and direction of the target, andthe reference time corresponds to a time at which internal reflectionsignals of the laser radar device are diminished.
 12. The apparatus ofclaim 11, wherein the monitoring signal is used by the laser radardevice to adjust a frequency of the pulsed laser beam to be closer to afrequency of a seed light used for generating the pulsed laser beam. 13.The apparatus of claim 11, wherein the photo-detecting means include, aphoto-detecting element that performs the coherent detection; and apre-amplifier operable to generate a first amplified signal byamplifying an output of the photo-detecting element.
 14. The apparatusof claim 13, wherein the photo-detecting means further include apost-amplifier operable to generate a second amplified signal byamplifying the first amplified signal, and the outputting means includesa microwave switch disposed between the preamplifier and the postamplifier, the microwave switch being operable to: direct the firstamplified signal to the signal processing unit as the monitoring signalbefore the reference time; and direct the first amplified signal to thepost-amplifier after the reference time, thereby allowing the secondamplified signal being output to the signal processing unit as thereception signal.
 15. The apparatus of claim 11, wherein the outputtingmeans include, a microwave amplifier operably connected to thephoto-detecting means, and gain control means for controlling a gain ofa signal output by the microwave amplifier.
 16. The apparatus of claim15, wherein the photo-detecting means include, a photo-detecting elementthat performs the coherent detection; and a pre-amplifier configured togenerate an amplified signal by amplifying an output of thephoto-detecting element, the amplified signal being sent to theoutputting means.
 17. The apparatus of claim 16, wherein the microwaveamplifier is a gain control amplifier operably connected to receive theamplified signal output by the pre-amplifier, an output of the microwaveamplifier further being directed to the signal processing unit, and thegain control means controls a gain of the gain control amplifier toincrease after the reference time.
 18. The apparatus of claim 16,wherein the gain control means is a microwave variable attenuatoroperably connected to receive the amplified signal output by thepre-amplifier, the microwave variable attenuator being controlled toattenuate the received amplified signal before the reference time, andthe microwave amplifier is a post amplifier operably connected toreceive an output of the microwave variable attenuator, an output of thepost amplifier being directed to the signal processing unit.
 19. Theapparatus of claim 11, further comprising a light variable attenuatoroperably connected to transfer the received scattered light to thephoto-detecting means, light variable attenuator being controlled toattenuate the received scattered light to a given threshold before thereference time.
 20. A method in a laser radar device, the laser radardevice being configured to transmit a pulsed laser beam toward a targetand process scattered light received from the target in a signalprocessing unit, the method comprising: coupling the output light of alight source with the received scattered light, the light source beingused to generate the pulsed laser beam; generating a first signal byperforming coherent detection on the coupled light to generate a firstsignal; outputting, before a reference time, the first signal to thesignal processing unit as a monitor signal; and outputting, after thereference time, a second signal to the signal processing unit as areception signal, the second signal being an amplified version of thefirst signal, wherein the reception signal is used by the signalprocessing unit to determine at least one of a speed and direction ofthe target, and the reference time corresponds to a time at whichinternal reflection signals of the laser radar device are diminished.21. The method of claim 20, further comprising: utilizing apre-amplifier to amplify a signal generated by the coherent detection;utilizing a post-amplifier to amplify an output of the pre-amplifier;and utilizing a microwave switch to perform the output steps, themicrowave switch being configured to direct the output of thepre-amplifier to the signal processing unit before the reference time,and to direct an output of the post-amplifier to the signal processingunit after the reference time.
 22. The method of claim 20, wherein theoutputting steps are performed by a gain control amplifier configured toreceive a signal generated as a result of the coherent detection, andthe method further comprises: controlling a gain of the gain controlamplifier to increase after the reference time.
 23. The method of claim20, further comprising: utilizing a microwave variable attenuator toattenuate a signal generated as a result of the coherent detection to agiven threshold before the reference time, wherein the outputting stepsare performed by a microwave amplifier configured to amplify an outputof the microwave variable attenuator.
 24. The method of claim 20,further comprising: utilizing a light variable attenuator to attenuatethe received scattered light to a given threshold before the referencetime, wherein the coupling step couples an output of the light variableattenuator to the output light of the light source.